Control circuits for energizing airport taxiway lighting are commonly designed to include certain basic components to permit desired control and to guarantee that certain safety requirements are met. The systems and control circuits are, basically, circuits which include relays, switches and one or more regulators. While the complexity is necessarily somewhat variable depending upon the size of the airport, such lighting systems have certain basic features in common. The lighting devices themselves are connected in series and two or more such series circuits are energized from a circuit selector switch box. The switch box is supplied with current from a current regulator which is controlled by a lighting control relay unit. The appropriate circuit selection in the lighting control relay unit is actuated by switches in a terminal control panel or, alternatively, by a pilot control unit in an aircraft which actuates a pilot control relay in the ground equipment.
Except for the pilot control unit in the aircraft, the above described equipment is located at various places on the ground. Normally the terminal control switches are physically separated, in different buildings, from the lighting control relay cabinet and the other components.
It has been found that a seriously disadvantageous and possibly dangerous set of conditions can arise in the use of this equipment resulting from the fact that the current regulator must be provided with an overvoltage responsive circuit which operates circuit breakers to shut the regulator off in the event of an external open circuit. It was observed that, under some conditions, an effort to turn on a specific taxiway series lighting circuit resulted in deenergization of the entire system, particularly when one lighting circuit had previously been energized. It was then necessary for someone at the terminal to become aware of, or be advised of, the problem and to go to the regulator location to manually reset the circuit breakers. This is not only inconvenient but can, for obvious reasons, be unsafe.
It is believed that the deenergization takes place because at least some of the relay contacts, which necessarily operated in make-before-break sequence, undergo some contact bounce, creating an open circuit which, even though transitory in nature, is detected as an open circuit by the regulator protection circuit, causing the circuit breakers to open. Furthermore, because of the other safety requirements of the system and the equipment components, the nature of the relays and the regulator protection cannot be changed.